Well cementing



Nov. 19, 1968 J. H. KOLAIAN ETAL WELL CEMENTING Filed Aug. 12, 1965 United States Patent O 3,411,579 WELL CEMENTIN G Jack H. Kolaian and Jack H. Park, Houston, Tex., as-

signors to Texaco Inc., New York, N.Y., a corporation of Delaware Filed Aug. 12, 1965, Ser. No. ::79,205 9 Claims. (Cl. 166-21) ABSTRACT OF THE DISCLOSURE A method for cementing a length of metal casing in a well filled with drilling mud wherein a D.C. electric potential -of from '0.01 to 2 volts is applied to the casing to cause the solid mud particles in the area to be cemented to migrate away from the casing by electrophoresis and the water content of the mud to migrate toward the casing by electroosmosis so that the surface of t he casing wall is clean and water wet. A cement slurry is then displaced into the treated region into contact with the clean surface of the casing. The application of a D.C. potential may be discontinued, the voltage may be reduced or it may be continued with a reversed polarity until the cement has set.

The present invention relates to a novel method for securing well casing in position within an uncased section of a well which has been drilled in the earth.

Oil and gas wells have been drilled in the earth for -many years by a rotating drill bit on the end of a string of drill tubng. Almost invariably a fluid drilling mud is employed by pumping it down through the drill string, and out through holes in the drill bit to pass up the surrounding annulus to the top of the well. Among the purposes of the drilling mud are to cool the drill bit and to provide a medium for carrying the cuttings out of the well to the surface of the earth. Normally drilling muds are made up of fluid suspensions of finely' divided clay particles in water, and may include various other ingredents for imparting de sired properties to the mud. For detaled information on drilling mud compositions reference can be had to hundreds of U.S. patents as well as many articles which have appeared in the technical journals and textbooks. For example, see composition And Properties Of Oil Well Drilling Fluids by Rogers, 3rd ed. 1963 (Gulf Publishing Co).

Upon completion of a well the drill string is removed and a length of steel casing is run into the mud-filled well to serve as the conduit for oil or gas to pass to the surface of the earth. Generally the casing is cemented within the well by depositing a water slurry of fluid portland cement in the annulus between the casing and the uncased wall of the well to hold the casing in centered position, as well as to seal off any formations which might introduce water or brine into the well casing instead of the desired oil or gas from the producng formation. Sometimes the well is drilled deeper by drilling through the cement in the bottom and then running smaller casings down through the first casing. When the casing is introduced into the well its outer walls become coated with the residual drilling mud which tends to be strongly adherent, so that when the cement is deposited in the annulus between the casing and the wall of the hole it will not contact the outer Wall of the casing and adhere thereto as firmly as desred.

In accordance with the present invention there is provided a novel method whereby the outer wall of the steel casing is substantially cleaned of drilling mud solids and other foreign matter such as oil, asphalt and Organic dispersants, by the application of an electrical voltage to the casing of a polarity the same as that of the solid particles in the drilling mud, thus causing the particles of mud to migrate away from the casing by electrophoresis and water to move to the casing by electroosmosis and leave its surface clean and water wet. This step is principally applicable to muds wherein the liquid phase s an electrolyte such as water, or an invert emulsion comprising water as the continuous phase and oil as the disperse-d phase. Then a cement slurry is positioned in the annulus around the outside of the casing and is allowed to set up to form a good bond between the resulting solid cement and the casing.

The principles of the invention will be described in detail hereinafter with reference to the accompanying drawings wherein:

FIG. 1 is a schematic vertical sectional view through an oil well showing an arrangement of apparatus for impressing a negative EMF on the Well casing prior to the introduction of the cement around the casing;

FIG. 2 is a vertical sectional view of the bottom portion of the oil well of FIG. 1, showing cement in position in the bottom of the well and around the casing; and

FIG. 3 is a vertical sectional view of the oil well of FIG. 1, but showing the sign of the applied electrical voltage reversed during the period of cement hardening.

Referring to FIG. 1, we have shown an oil well W having a short length of surface casing S at its upper end, and a length of production casing C extending cent rally down into the well through the top of the casing S into the unlined section of the well.

The well W is full of fluid drilling mud M which surrounds the casing C and extends to the surface of the earth. A slurry of portland cement P is supplied through a conduit 11, fills the casing C, and is pumped downwardly to displace drilling mud. Before and after the cement P leaves the lower end of casing C, a negative electrical potential is applied to the casing at its upper end by a suitable current source such as a D.C. electrical generator G. The positive side of generator G is connected to a ground electrode E embedded in the earth a short distance away from the top of the well. Ground electrode E may be a metal rod or tube installed especially for the purpose, or may be the casing of a nearby oil well in an oil field. A potential drop within the range of 0.01 to '5 volts should be used at the surface of the casing. At potential drops greater than 2 volts water decomposition may occur and hydrogen be evolved. Where hydrogen liberation is apt to be detrimental, therefore, t is advantageous for 2 volts to be the upper limit. As the potential drop increases above 0.01 volt the rate of water accumulation at the surface of the casing increases.

As a result of impressing a voltage on the casing C the solid particles of the mud which have been adsorbed on the surface of the casing are caused to migrate outwardly through the liquid phase of the mud toward the earth wall of well W, and water to move to the casing surface, leaving the outer surface of casing C quite clean and wet. As the portland cement slurry P passes downwardly in casing C it displaces the mud upwardly out of the hole through a conduit 13 until, as shown in FIG. 2, the cement P leaves the bottom of the casing and rises up into the annulus surrounding the casing to the position Where the cement seal is to be completed. When the proper level has been reached, as 'at 15 in FIG. 2 pumping of 'cement is stopped and the cement is then allowed to set in intimate contact with both the outer surface of the casing and the wall of the well to produce a good strong seal. Of course it is not necessary to have a full column of cement slurry as it is conventional to introduce a slug of cement slurry sufficient for the job and follow this with water or other fluid which can be pumped against the top of the cement column.

In order to assure a satisfactory seal either of three procedures can be followed. In the first procedure, the application of electrical voltage to casing C is discontinued entirely and the cement slurry sets in a normal unaffected manner, If application of the negative voltage were continued at the original voltage it would tend to cause the solid particles of portland cement to migrate away from the casing and thus produce a relatively Weak seal at the surface of the casing.

In the second procedure, a negative voltage is maintained on the casing, but at a reduced value such that the flow of water toward the casing does not occur. Thus the water content of the cement sheath adjacent the casing is maintained at a constant value, to ofiset the tendency for water to flow out of the cement into the surrounding earth formation at too high a rate, thus producing a final set cement of high strength. A potential drop at the casing of 0.1 volt or less, for example 0.01- 0.l volt, is adequate for this purpose.

In the third procedure, as shown in FIG. 3, the polarity of the casing C is reversed by connecting the positive terminal of the generator G to the casing and the negative terminal to the ground electrode E, and applying an appropriate voltage within the above range. By so doing, particles of cement in the slurry migrate toward the casing C and assure the presence of sufficient solid cement ingredients at the casing to provide a good seal. Application of the voltage normally is continued until the cement has set.

The general principles of oil well cementing are well known and are adequately described in an article appearing in the Petroleum Engneer for May 1961 beginning at p. B-91, and continuing in the June 1961 issue beginning at p. B-28.

The invention has been described as carried out by the initial application of a negative voltage to the oil well casing, and this indeed is the most generally useful type of operation because in most drilling muds the solid particles are negatively charged and will then be repelled from the casing. However, in the event a drilling mud is employed wherein the solid particles are positively charged, it is necessary to connect the positive side of the source of voltage to the casing and the negative side to the ground electrode.

For Simplicity of description, the method has been described as applied to a single oil well casing C centrally located within a well W. However, it is evident that the same principles can be applied to multiple completions wherein two or more steel tubes positioned side by side are located within the well W. When cementing multiple completions the voltage is applied to each casing being cemented at any given time, to assure an adequate seal of the solidified cement to the several casings. Likewise when several concentric casings of different diameters and extending to different depths are employed.

The method described in detail above is based upon the 'phenomena of electroosmosis and electrophores's, which are dened as the migration of Suspended solid, liquid or gaseous colloidal particles under the influence of an external electromotive force. Another application of this phenomenon for freeing pipe jammed in a well is described in U.S. Patent 2,372,575 which was granted Mar. 27, 1945 to John T. Hayward. Similar principles were employed by H. T. Byck in U.S. Patent 2,2l7,857

granted Oct. 15, 1940 for the removal of mud sheaths from the wall of a well. I-Iowever, none of the known prior art was concerned with improving a cement bond surrounding a steel casing in a well.

Tests have shown that confirm the following resistance relationship between any *two lengths of pipe used as electrodes in a simple D.C. circuit:

P lu

R ab

4 where P=apparent resistivity of the formation between the electrodes. h =length of the shorter electrode. D distance between the electrodes. a and b=radii of the electrodes.

From Equation 1 one obtains the current I and the power W as follows:

Example I A steel electrode E comprising an oil well casing 6 inches in diameter and 1000 feet long is located feet away from a casing C to be cemented, which also is 6 inches in diameter. The earth formation has an apparent resistivity of 3 ohm ft.

While the portland cement is pumped down into the casing C and up into its final position in the annulus surrounding the casing a negative voltage of 2 volts is impressed on casing C, with a current of 327 amps and a power of 654 watts.

When the cement is in final position, the current is discontinued and the cement is allowed to set.

Example II Cementing is accomplished as in Example I, but with the additional step of maintaining a reduced voltage of 0.1 volt on the casing C, and a current of 16.3 amps, while the cement sets so as to offset the tendency of water to flow out ot the cement and into the surrounding earth.

Example III A steel electrode E comprising an oil well casing 6 inches in diameter and 2000 feet long is located 300 feet away from a casing C 12 inches in diameter, the earth formation again having a resistivity of 3 ohm ft.

During displacement of the drilling mud and positioning of a portland cement slurry a negative voltage of 0.01 volt is impressed on casing C, with a current of 3.12 amps and a power of .0312 watt. Then the current is discontinued and the cement allowed to set.

In still another operation employing the principles of electroosmosis and electrophoresis, no cement is used but the drilling mud itself is treated to hold the casing in place. With the annulus between the casing and the earth containing drilling mud, a positive voltage, say of 0.1 volt, is applied to the casing to cause the negative particles of mud solids to -pack around the casing quite tightly and hold it firnly in place. Of course where the particles are positively charged, a negative voltage is applied.

A combination way of doing this involves providing .the casing C with an electrically insulating coupling or joint (eg. of phenol formaldehyde resin) above the zone where portland cementing is required. A negative D.C. voltage is then applied to the metal casing below the coupling while portland cement is pumped into 'place down the casing and into the annulus to a position below the coupling, displacing mud upwardly. At the same time a p0sitiveD.C. voltage is applied to the metal casing above the coupling to cause the mud above the coupling to pack tightly around the casing.

The simultaneous application of positive and negative voltages can be accomplished by separate conductor wires leading from suitable sources at the surface through the casing to connections thereon.

Obviously, many modifications and variations of the V invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are. indicated in the appended claims.

We claim:

1. A method .for cementing a length of metal casing in a well wherein said casing is initially surrounded by a mass of fluid drilling mud comprising,

applying to said casing a unidirectional electrical voltage within the range of about 0.01 to 2 volts and having a polarity the same as that of the solid particles in the drilling mud to cause said particles to be repelled from said casing and leave the outer wall thereof in a clean condition;

displacing said drilling mud away from a section of said casing which is to be cemented, while maintaining said application of voltage;

depositing a cement slurry at said section between the outer surface of said casing and the wall of said well; and

permitting said cement slurry to set in contact with said casing and the wall of said well.

2. A method in accordance with claim 1, also comprisin-g the step of discontinuing the application of said voltage to said casing while said cement slurry sets.

3. A method in accordance with claim 1, also comprising, while said cement is permitted to harden, applying to said casing a voltage of a sign opposite that initially applied.

4. A method in accordance with claim 1 wherein said cement slurry is deposited at said section by pumping a cement slurry down through said casing and out into said well;

and forcing said drilling mud out of said well by said cement so that said cement rises around the outside of said casing into said section.

5. A method in accordance with claim 1 wherein the voltage applied to said casing is negative.

6. The method in accordance with claim 1 wherein a voltage of opposite sign to that applied to said casing, is

applied to a ground electrode embedded in the ground at a position spaced from the top of said well.

7. A method in accordance with claim 3 wherein a negative voltage is applied to said casing while said cement displaces said drilling mud from said Well; and

wherein a positive voltage is applied to said casing while said cement is permitted to harden around said casing;

8. A method for securing a length of metal casing in a well wherein said casing is surrounded by a mass of fluid drilling mud comprisng applying; to said casing a unidirectional electrcal potential in the range of 0.01 to 2 volts and having a sign opposite that of the solid particles in the drilling mud to cause said particles to be attracted to said casingthereby packing tightly around said casing.

9. A method .for securing a length of metal casing in a well wherein said casing is surrounded by a mass of fluid drilling mud, comprising providing electrical insulation between a lower part and an upper part of said casing; applying' a negative voltage to said lower part while introducing a cement slurry to displace said mud upwardly to a position above said insulation;

applying a positive voltage to said upper part to cause the solid particles in said mud to pack tightly around said upper part;

and permitting said cement to set.

References Cited UNITED STATES PATENTS 2,211,'696 8/ 1940 Irons 204- 2,217,857 10/ 1940 Byck 166-21 X 2,283,206 5/1942 Hayward 166-21 X 2,372,575 3/1945 Hayward 204--180 2,625,374 1/1953 Neuman 204-180 X 2,799,641 7/1957 Bell 204-180 3,189,088 6/ 1965 Cronberger 166-25 NILE C. BYERS, JR. Primary Exam'ner. 

